The present invention relates to an X-ray photographic equipment in which an X-ray television is utilized, and more particularly to an X-ray photographic equipment for carrying out digital subtraction angiography (DSA) in which video signals to be processed are digitalized.
When a subject is to undergo DSA in two or more areas of his body, and when these areas are disposed along the direction of flow of a constrast medium, it is most common to carry out DSA with either of the following two techniques, in either of which the constrast medium has only to be injected once for all so as to avoid a cardiodynamic burden which would be imposed on the heart of the subject if the constrast medium were injected in each area:
The first technique comprises the steps of taking a pre-constrast image in one area after another, injecting a contrast medium, taking a contrast image in one area after another, and subtracting the pre-contrast image from the contrast image for each area. A difference obtained from this subtraction represents an image of the contrast medium alone, i.e. an image of the blood vessel alone, the background of which has vanished. Twice in all, an examination table, on which the subject is lying, is moved relative to an image pickup system including an X-ray tube, i.e. once before the injection of the contrast medium and once after the injection thereof.
The second technique begins with taking a pre-contrast image in an area disposed at the upstream side of the flow of a contrast medium. The pre-contrast image is taken before the contrast medium reaches the area. Subsequently, when the contrast medium reaches the area, a contrast image is taken in that area. Then an area disposed at the downstream side is allowed to come into view of the image pickup system, and a pre-contrast image is taken in that area before the contrast medium reaches the area. Subsequently, when the contrast medium reaches the area, a contrast image is taken in that area. Thus the movement of the subject relative to the image pickup system precedes the flow of the contrast medium. In each of a plurality of areas, both a pre-contrast image and a contrast image are taken, and the former is subtracted from the latter.
The first technique is characterized in that, after the first movement of the examination table for the purpose of taking pre-contrast images in a plurality of areas, the examination table is returned to its starting position. The trouble is that, at the time of the second movement, it is difficult to stop the examination table exactly in the same positions as those in which the examination table was stopped at the time of the first movement. Consequently an artefact caused by positional deviation is liable to appear. Even if the examination table per se is free from positional deviation, the body of the subject is liable to deviate from its initial posture in the course of the long interval between the time when a pre-contrast image is taken in an area and the time when a contrast image is taken in the same area. Consequently an artefact caused by postural deviation is also liable to appear.
In the second technique, the examination table has only to be moved once from the uppermost to the downmost side of the contrast medium stream. There is no possibility, therefore, that an artefact caused by positional deviation will appear. It is difficult, however, to start moving the examination table at an opportune moment. The contrast medium may have already reached a second area disposed at the downstream side if the second area is late in coming into view of the image pickup system. In that event, the pre-contrast image will include an image of the contrast medium (i.e. an image of the blood vessel), which will be lost when this pre-contrast image is subtracted from a contrast image. Consequently the portion where the blood vessel exists turns white on an image obtained from such subtraction. As a matter of course, one may contend that the problem of this kind does not arise if the movement of the examination table is started at an opportune moment so that the second area may come into view of the image pickup system before the contrast medium reaches the area. However, this contention does not fit in with the reality, because there is great individual variation as to the velocity of flow of the contrast medium (i.e. the velocity of blood flow).